NEPHRONS

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INTRODUCTION
Nephron is defined as the structural and functional unit of kidney. Each kidney consists of 1 to 1.3 millions of nephrons.
The number of nephrons starts decreasing after about 45 to 50 years of age at the rate of 0.8% to 1% every year. Each nephron is formed by two parts:
1. A blind end called renal corpuscle or Malpighian corpuscle
2. A tubular portion called renal tubule.
Diagram of the nephron
 
 
RENAL CORPUSCLE
Renal corpuscle or Malpighian corpuscle is a spheroidal and slightly flattened structure with a diameter of about 200 μ. Function of the renal corpuscle is the filtration of blood which forms the first phase of urine formation. Renal corpuscle is situated in the cortex of the kidney either near the periphery or near the medulla.

CLASSIFICATION OF NEPHRONS 
Based on the situation of renal corpuscle, the nephrons are classified into two types:
1. Cortical nephrons or superficial nephrons: Nephrons having the corpuscles in outer cortex of the kidney near the periphery. In human kidneys, 85% nephrons are cortical nephrons.

 2. Juxtamedullary nephrons: Nephrons having the corpuscles in inner cortex near medulla or corticomedullary junction.
Diagram showing the classifications of the nephron
 
STRUCTUREOFRENALCORPUSCLE
Renal corpuscle is formed by two portions: 1. Glomerulus
2. Bowman capsule.

GLOMERULUS 
Glomerulus is a tuft of capillaries enclosed by Bowman capsule. It consists of glomerular capillaries interposed between afferent arteriole on one end and efferent arteriole on the other end. Thus, the vascular system in the glomerulus is purely arterial.
DIAGRAM OF THE RENAL CORPUSCLES 
Glomerular capillaries arise from the afferent arteriole. After entering the Bowman capsule, the afferent arteriole divides into 4 or 5 large capillaries.

Each large capillary subdivides into many small capillaries. These small capillaries are arranged in irregular loops and form anastomosis. All the smaller capillaries finally reunite to form the efferent arteriole, which leaves the Bowman capsule.

Diameter of the efferent arteriole is less than that of afferent arteriole. This difference in diameter has got functional significance.

FUNCTIONAL HISTOLOGY 
Glomerular capillaries are made up of single layer of endothelial cells, which are attached to a basement membrane. Endothelium has many pores called fenestrae or filtration pores.

Diameter of each pore is 0.1 μ. Presence of the fenestra is the evidence of the filtration function of the glomerulus.


BOWMAN CAPSULE 
Bowman capsule is a capsular structure, which encloses the glomerulus. It is formed by two layers: 
i. Inner visceral layer 
ii. Outer parietal layer.

Visceral layer covers the glomerular capillaries. It is continued as the parietal layer at the visceral pole.

Parietal layer is continued with the wall of the tubular portion of nephron. The cleft like space between the visceral and parietal layers is continued as the lumen of the tubular portion.

Functional anatomy of Bowman capsule resembles a funnel with filter paper. Diameter of Bowman capsule is 200 μ

FUNCTIONAL HISTOLOGY 
Both the layers of Bowman capsule are composed of a single layer of flattened epithelial cells resting on a basement membrane.

Basement membrane of the visceral layer fuses with the basement membrane of glomerular capillaries on which the capillary endothelial cells are arranged. Thus, the basement membranes, which are fused together, form the separation between the glomerular capillary endothelium and the epithelium of visceral layer of Bowman capsule.
Diagram of epithelial cell of Bowman capsule
 
Epithelial cells of the visceral layer fuse with the basement membrane but the fusion is not complete.
Each cell is connected with basement membrane by cytoplasmic extensions of epithelial cells called pedicles or feet.
These pedicles are arranged in an interdigitating manner leaving cleft-like space in between. The cleft-like space is called slit pore. Epithelial cells with pedicles is called podocytes.

TUBULAR PORTION OF NEPHRON
Tubular portion of nephron is the continuation of Bowman capsule. It is made up of three parts:
1. Proximal convoluted tubule
2. Loop of Henle
3. Distal convoluted tubule.

 PROXIMAL CONVOLUTED TUBULE
Proximal convoluted tubule is the coiled portion arising from Bowman capsule. It is situated in the cortex. It is continued as descending limb of loop of Henle.

 Length of proximal convoluted tubule is 14 mm and the diameter is 55 μ. Proximal convoluted tubule is continued as loop of Henle.

FUNCTIONAL HISTOLOGY 
Proximal convoluted tubule is formed by single layer of cuboidal epithelial cells. Characteristic feature of these cells is the presence of hairlike projections directed towards the lumen of the tubule. Because of the presence of these projections, the epithelial cells are called brush-bordered cells.
LOOP OF HENLE
Loop of Henle consists of:
i. Descending limb
ii. Hairpin bend
iii. Ascending limb.

DESCENDING LIMB 
Descending limb of loop of Henle is made up of two segments:
a. Thick descending segment 
b. Thin descending segment.

THICK DESCENDING SEGMENT 
hick descending segment is the direct continuation of the proximal convoluted tubule. It descends down into medulla.

It has a length of 6 mm and a diameter of 55 μ. It is formed by brush bordered cuboidal epithelial cells.

THIN DESCENDING SEGMENT 
Thick descending segment is continued as thin descending segment. 

It is formed by flattened epithelial cells without brush border and it is continued as hairpin bend of the loop.
 
 ii. Hairpin Bend
Hairpin bend formed by flattened epithelial cells without brush border and it is continued as the ascending limb of loop of Henle.
iii. Ascending Limb


Ascending limb or segment of Henle loop has two parts:
 a. Thin ascending
b. Thick ascending segment.

Thin ascending segment
Thin ascending segment is the continuation of hairpin bend. It is also lined by flattened epithelial cells without brush border.

 Total length of thin descending segment, hairpin bend and thin ascending segment of Henle loop is 10 mm to 15 mm and the diameter is 15 μ. Thin ascending segment is continued as thick ascending segment.

Thick ascending segment
Thick ascending segment is about 9 mm long with a diameter of 30 μ. Thick ascending segment is lined by cuboidal epithelial cells without brush border.

The terminal portion of thick ascending segment, which runs between the afferent and efferent arterioles of the same nephrons forms the macula densa. Macula densa is the part of juxtaglomerular apparatus.
Thick ascending segment ascends to the cortex and continues as distal convoluted tubule.
Length and Extent of Loop of Henle
Length and the extent of the loop of Henle vary in different nephrons:
 i. In cortical nephrons, it is short and the hairpin bend penetrates only up to outer medulla

In juxtamedullary nephrons, this is long and the hairpin bend extends deep into the inner medulla.
 In some nephrons it even runs up to the papilla.

DISTAL CONVOLUTED TUBULE
Distal convoluted tubule is the continuation of thick ascending segment and occupies the cortex of kidney. It is continued as collecting duct.

The length of the distal convoluted tubule is 14.5 to 15 mm. It has a diameter of 22 to 50 μ.

FUNCTIONAL HISTOLOGY 
Distal convoluted tubule is lined by single layer of cuboidal epithelial cells without brush border. Epithelial cells in distal convoluted tubule are called intercalated cells (I cells).

COLLECTING DUCT
Distal convoluted tubule continues as the initial or arched collecting duct, which is in cortex. The lower part of the collecting duct lies in medulla. Seven to ten initial collecting ducts unite to form the straight collecting duct, which passes through medulla.

Length of the collecting duct is 20 to 22 mm and its diameter varies between 40 and 200 μ. Collecting duct is formed by cuboidal or columnar epithelial cells.

FUNCTIONAL HISTOLOGY 
Collecting duct is formed by two types of epithelial cells:
1. Principal or P cells
 2. Intercalated or I cells.

These two types of cells have some functional significance.

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